1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a laser beam printer adopting an electrophotographic system, in particular, a frame construction of an apparatus main body.
2. Description of the Related Art
With regard to an image forming apparatus adopting an electrophotographic system, there has been proposed a construction in which rigidity of a frame of an apparatus main body (hereinafter referred to as main body frame) is enhanced for the purpose of performing image formation of higher image quality. When the rigidity of the main body frame is low, positional deviation of optical components such as the photosensitive drum and the laser scanner arranged within the main body frame is caused by the influence of vibration, etc. at the time of driving of the apparatus main body, resulting in image failure such as image distortion or misregistration.
FIG. 9 schematically illustrates a construction of the main body frame of a conventional image forming apparatus. As illustrated in FIG. 9, the conventional main body frame is equipped with a front side plate 50 and a rear side plate 51 formed by sheet metal stamping through press working; between the two side plates, there are provided support members 52, 53, 54, and 55 and, further, reinforcing members 56 and 57. The support members 52, 53, 54, and 55 are fastened to the side plates by screws or welding.
However, in a recent image forming apparatus, of which high image quality performance is required, the above-mentioned conventional main body frame is rather inadequate in terms of rigidity, thus a high quality image cannot be formed on a sheet material. If, in order to enhance the rigidity of the main body frame, a reinforcing member is provided in the main body frame or the thickness of each member constituting the main body frame is increased, the production cost of the main body frame is increased.
In this regard, Japanese Patent Application Laid-Open No. 2001-66840 discloses a construction in which the rigidity of the main body frame is enhanced by forming a box-shaped closed space at the bottom of the main body frame by a support member mounted between a front side plate and a rear side plate.
With this construction, there is no need to provide a reinforcing member on the main body frame, so it is possible to enhance the rigidity of the main body frame while suppressing an increase in production cost.
However, when a box-shaped closed space is formed at the bottom of the main body frame as in the case of the above-mentioned conventional example, the interior of the closed space constitutes a dead space, resulting in an increase in size of the apparatus main body.
Further, the front side plate and the rear side plate are twisted with respect to each other around the support member forming the closed space. Thus, in the construction in which the closed space is provided at the bottom of the main body frame, the twisting of the side plates occurs to a large degree at a position spaced apart from the bottom portion of the main body frame.
As a result, when an optical component such as a laser scanner is mounted to the front side plate and the rear side plate at a position spaced apart from the bottom portion of the main body frame, twisting of the side plates causes positional deviation of the optical component, thus a high quality image cannot be obtained.